1. Field of the Invention
The present invention relates to nonreciprocal circuit devices and to communication devices. More particularly, the present invention relates to a nonreciprocal circuit device, such as an isolator and a circulator, for use in the high frequency band such as the microwave band, and to a communication device incorporating the same.
2. Description of the Related Art
Nonreciprocal circuit devices, such as lumped constant isolators and lumped constant circulators, are used in communication devices such as cellular phones, taking advantage of the characteristics that the devices exhibit a very small attenuation with respect to the direction of signal transmission, and a very large attenuation with respect to the reverse direction.
As shown in an equivalent circuit in FIG. 8, a lumped constant isolator typically includes three central conductors L disposed on a magnetic body (ferrite) so as to intersect with one another, matching capacitors C0 connected between the ground and respective ports P1, P2 and P3 of the central conductors L, and a terminating resistor R connected to the port P3, DC magnetic field Hex being applied to the magnetic body and the central conductors. The magnetic body is indicated by a broken line in FIG. 8.
In a typical communication device, amplifiers incorporated in the circuit inevitably generates some distortion, causing spurious radiations such as the second and the third harmonic components of the fundamental wave. Rules and standards are provided, dictating the spurious radiations be kept below a particular level. The spurious radiations can be prevented with amplifiers with good linearity; however, such amplifiers are rather expensive. A common alternative method is to provide a filter or the like so as to attenuate undesired frequency components. However, use of such filters increases cost and the size of the communication device, and also causes loss.
Furthermore, in the communication device, isolators and circulators are used for stable operation and protection of amplifiers in the circuit. Particularly, lumped constant isolators and lumped constant circulators exhibit band-pass filter characteristics in the forward direction, attenuating signals, even in the forward direction, in frequency bands off the pass band. However, the conventional nonreciprocal circuit device having the basic construction as shown in FIG. 8 has failed to provide sufficient attenuation within the undesired frequency band.
Japanese Unexamined Patent Application Publications Nos. 10-93308 and 10-79607 each disclose a nonreciprocal circuit device which provides a large attenuation in the frequency band of spurious radiations, particularly the second and third harmonic components of the fundamental wave. In the nonreciprocal circuit devices disclosed therein, in addition to the construction shown in FIG. 8, an inductor is provided on an input port or an output port, and a capacitor is externally connected, thereby constituting a low-pass filter. Thus, the components of the undesirable frequency band is attenuated to reduce the spurious radiations, the overall communication device can be constructed more compact compared to an arrangement in which a separate filter is externally provided.
However, in the nonreciprocal circuit devices disclosed in the Japanese Unexamined Patent Application Publications Nos. 10-93308 and 10-79607, one inductor and one or two capacitors are required to constitute a low-pass filter, raising problems that the number of parts is increased, and that setting of the inductance and capacitance values is sensitive, thus inhibiting reduction in size and in cost. That is, addition of discrete capacitors increases the number of parts and the cost. On the other hand, use of a mounting board and matching capacitors involves restrictions regarding the characteristic values of each of the components, rendering the design difficult. Furthermore, the capacitors for filtering purpose provided parallel to the matching capacitors results in an increased size. In addition, an inductor of a relatively large inductance value is required to constitute the low-pass filter.
Accordingly, it is an object of the present invention to provide a compact, inexpensive nonreciprocal circuit device which provides a large attenuation in a particular frequency band, and a communication device incorporating the same.
To this end, a nonreciprocal circuit device according to the present invention includes a magnetic unit which receives DC magnetic field, said magnetic unit having a plurality of central conductors disposed so as to intersect with one another; and a series resonance circuit constituted of an inductor and a capacitor, having a resonance frequency higher than the operating frequency of the nonreciprocal circuit device, connected between the ground and a port section of one of the plurality of central conductors used as an input or output port. More specifically, an inductor is connected in series to a conventional matching capacitor so as to form a series resonance circuit on the port section of one of the central conductors used as an input or output port. Whether the series resonance circuit is connected to either the input port or the output port, or to both the input and output ports, is determined in accordance with the desired shape (size), attenuation, etc.
In the above construction, the series resonance circuit constituted of the inductor and the capacitor forms a trap having a pole at a frequency higher than the operating frequency of the nonreciprocal circuit device, providing a large attenuation in the frequency band higher than the operating frequency, and attenuating undesired radiations of the second and third harmonic components of the fundamental wave (the operating center frequency).
Thus, by having the series resonance circuit serve as both a matching circuit and a band stop filter, a separate filter for preventing the undesired radiations, components of the filter, an LC series resonance circuit, etc. need not be externally provided. Thus, the number of parts can be reduced, and the nonreciprocal circuit device and the communication device can be implemented in a reduced size and in a reduced cost.
Furthermore, in accordance with the construction, the values of the capacitance and the inductance can be reduced compared with those of the low-pass filter in the Japanese Unexamined Patent Application Publication No. 10-93308, and the nonreciprocal circuit device can be further reduced in size.
Generally, the attenuation of the second harmonic component is smaller than that of the third harmonic component in the nonreciprocal circuit device, and thus, the undesired radiations are most effectively suppressed when the resonance frequency of the series resonance circuit is in the vicinity of the frequency of the second harmonic component. This resonance frequency is preferably between the frequency of the fundamental wave and that of the third harmonic component.
The series resonance circuit can be formed without increasing the number of parts by integrally forming the inductor constituting the series resonance circuit with the central conductors, and in addition, the cost can be reduced.
The communication device of the present invention includes the nonreciprocal circuit device having the abovedescribed characteristics. A compact, inexpensive communication device which provides preferable characteristic.